The prior art drilling apparatus are of two kinds, differing in where in the drill string the power to rotate the drilling bit is applied. In ordinary rotary drilling, a long string of drill pipe rotates a drill bit by rotation of the drill pipe from the surface.
An alternative method mounts a motor at the end of the drill string adjascent the bit. The motor is operated by the drilling fluid which is pumped down the drill pipe, through the motor and drill bit and circulated to the surface carrying the detritus generated by the bit as it advances to form the bore hole.
One form of such motor is a turbine. This technology is well known and will not need further explanation to those skilled in this art. The performance of the turbine at constant drilling fluid pumping rate (gallons of drilling fuid per minute--gpm) is a function of the rpm (revolutions per minute) of the turbine.
Thus with the bit off bottom, as when the string is lifted from bottom, the turbine runs faster and faster, rpm rapidly increases, with less and less torque until a balance between centrifugal and frictional forces occurs. When the bit is placed in drilling position, the braking effect of the bit rotating on bottom in drilling position at constant drilling input rate (gpm) results in an increase in the torque with a reduction in rpm of the drill.
Furthermore, at a constant gpm, the horsepower output and turbine efficiency passes through an optimum as the rpm increases.
In common practice, both in ordinary rotary drilling as well as in operations with an inhole turbine a portion of the weight of the drill string is imposed on the drill bit so as to obtain as rapid an advance of the drill as is practical for the formations to be drilled.
As in ordinary rotary drilling, so also in the case of turbine drilling, the weight imposed on the drill depends on the tension in the drilling lines from which the drill string is suspended in the derrick.
Excessive increase in tension in the drilling lines not only reduces the load on the bit and thus the drilling rate but if carried too far may cause a rupture of the drill pipe. In the case of turbine drilling, the reduction of load on the bit by increase in the tension in the drilling lines increases the rpm of the turbine, the gpm having been held intact. The increase in rpm, with drilling fluid gpm held substantially constant causes a substantial decrease of torque and thus drill bit advance. The rpm may also move from the desirable range of rpm for optimum efficiency and horse power output.
On the other hand, excessive decrease in tension results in an excessive load on the bit, which in the case of turbine drilling causes a reduction in rpm.
In order to avoid such excess of weight variation, it is common practice to control the braking action of the draw-works (winch) to control the tension in the drilling lines as measured by a tensometers mounted on the drilling lines.
Such controls may be done manually by control of the brake on the draw works, or automatically whereby the brake is automatically set responsive to a signal from the tensometer.
The rpm at which the turbine operates is thus an important criterion of the performance not only of the efficiency of the turbine but also of the proper performance of the drilling rig.
As drilling progresses, and the hole deepens, the brake must allow the drill string to advance at the desired rate while the tension in the lines maintains the desired weight on the bit.
In ordinary rotary drilling this is accomplished either by manual or automatic control of the draw works brake to advance the drill string while maintaining the tension so as to maintain the weight on the drill in the desired range.
The prior art control of the performance of a turbine driven bit by control of the weight of the bit relied on the braking action of the bit which is responsive to the weight imposed on the bit.
The braking action of the bit is affected not only by the weight on the bit, but also by the nature of the formation being drilled.
Reliance on the weight to control turbine performance is thus not entirely sufficient.